This invention relates to novel, improved high quality brominated styrenic polymers eminently well suited for use as flame retardants in thermoplastic polymer compositions.
Brominated polystyrenes are well established as flame retardants for use in thermoplastics, e.g., polybutylene terephthalate, polyethylene terephthalate and nylon (a.k.a. polyamides). Recently, interest has been shown for expanding their use to syndiotactic polystyrene and polycyclohexylene dimethylene terephthalate. Generally, brominated polystyrenes are produced by a reaction between polystyrene and a brominating agent (e.g., bromine or bromine chloride) in the presence of a solvent (e.g., dichloroethane) and a Lewis acid catalyst. Heretofore the art has proffered many processes which are claimed to produce a superior brominated polystyrene. See U.S. Pat. Nos. 4,200,703; 4,352,909; 4,975,496 and 5,532,322.
Despite these efforts, previously-known brominated polystyrene flame retardants remain deficient in certain properties which translate into deficient performance of thermoplastic polymer blends in which they arc used when the blends are subjected to polymer processing conditions.
To better understand some of the reasons for these deficiencies, it is helpful to consider some of the structural characteristics of previously known brominated polystyrenes. To begin with, the bromine content of a brominated polystyrene is the sum of (1) the bromine which is substituted onto the aromatic portions of the polymer, (2) the bromine which is substituted onto the aliphatic portion(s) of the polymer, e.g., the polymer backbone or alkyl substitution which is present due to alkylation of the aromatic portion of the polymer, and (3) any ionic bromine present, e.g., sodium bromide. The alkylation of aromatic rings in the brominated polystyrene is catalyzed by the Lewis acid catalyst used in producing the brominated styrenic polymer, and the reaction solvent (usually a 1-3 carbon atom dihaloalkane) serves as the alkylating agent. The bromine for (1) is referred to herein as aromatic bromide, while the bromine for (2) is referred to as aliphatic bromide. Even though ionic bromine can contribute to the total bromine content, its contribution to the total bromine content is small. Nevertheless, as pointed out in U.S. Pat. No. 5,328,983, ionic impurities in brominated polystyrene may degrade polymer formulations in respect to their ultimate electrical properties, and also may result in corrosion of processing equipment or in the corrosion of metallic parts in their end-use applications.
The chlorine content of brominated polystyrenes is credited to chlorine which, like the bromine, is chiefly part of the polymer structure as an aromatic and/or an alkyl chloride. The use of bromine chloride as the brominating agent is the largest contributor to the chlorine content. However, chlorinated solvents and/or chlorine-containing catalysts used in the production of the brominated polystyrene may also contribute to the chlorine content of the brominated polystyrene.
The aliphatic halide content of the brominated polystyrene is not desirable as aliphatic halide is not as thermally stable as aromatic halide and, thus, aliphatic halide can be easily converted to hydrogen halide, e.g., HBr or HCl, under normal end-use processing conditions. To evaluate brominated styrenic polymers for their tendencies to release hydrogen halide under thermal processing conditions, use is made hereinafter of the method described in U.S. Pat. No. 5,726,252 and referred to therein as the Thermal Stability Test Procedure. In essence, this method indicates the content of halogen atoms in the brominated polystyrene that is not bonded directly to the aromatic rings and thus is more readily released from the polymer when at elevated temperature.
Apart from whether the halide is present as an aromatic or aliphatic halide, it is also desirable to minimize the total chlorine content of the brominated polystyrene as chlorine is not as efficacious or as stable a flame retardant constituent as is bromine.
Total halogen content, especially total bromine content, of the brominated polystyrene is another important consideration. Pyro-Chek.RTM. 68PB brominated polystyrene flame retardant (Ferro Corporation) is reported to have a total halogen content of about 67 wt %. U.S. Pat. Nos. 5,637,650 and 5,726,252 of Ferro Corporation report that Pyro-Chek.RTM. 68PB flame retardant has 3000 to 6000 ppm of backbone halogen, 5000-6000 ppm being typical. And when Pyro-Chek.RTM. 68PB flame retardant was subjected to the Thermal Stability Test in our laboratories, it evolved 1960 ppm HBr. In order to provide a brominated polystyrene that melt blends more easily and efficiently than Pyro-Chek.RTM. 68PB flame retardant, another product with a lower total halogen content was developed by Ferro Corporation, namely, Pyro-Chek.RTM. 60PB brominated polystyrene flame retardant. This product, which has a total bromine content of about 61 wt %, is reported to melt at a lower temperature and to flow more easily during compounding and processing operations than Pyro-Chek.RTM. 68PB flame retardant. However, these improvements in melt flow are achieved with a reported concomitant reduction in aromatic bromine content of 10%. Thus, if anything, the 10% reduction in aromatic bromine content portends at best no improvement in terminal stability and at worst a reduction in thermal stability as compared to Pyro-Chek 68PB flame retardant.
It would be especially desirable and of considerable advantage, if a brominated styrenic polymer, e.g., brominated polystyrene, could be provided that has both improved melt flow characteristics and improved thermal stability. It would also be of great advantage if these improvements could be achieved without material sacrifice of other important properties, and if possible with concomitant provision of other desirable properties such as low ionic bromide content, minimal (if any) chlorine content, and desirable color and odor properties.